 Solution

Practice contributes substantially to compensatory performance after sensory loss, but evidence for selective crossmodal recruitment and causal changes in neural contribution goes beyond an account based solely on accumulated task repetitions. Practice and plasticity are not mutually exclusive: practice can induce neural change, while deprivation can change which circuits practice recruits.

The first requirement is to specify the claimed enhancement. Better tactile discrimination, peripheral visual localization and visual motion detection are different outcomes. Accuracy can improve because of sensory precision, attention, memory, a decision criterion or a slower response. There is no inference from success on one selected task to a universal sharpening of the remaining senses. Sensory loss also differs in onset, completeness, cause and duration. Early blindness, late blindness and temporary blindfolding cannot be treated as equivalent experiments.

A practice account predicts learning with repeated use, specificity to trained inputs and possible transfer to related tasks. A deprivation account predicts changes associated with absent input even when practice is matched. Both can operate, and a statistical difference between groups cannot by itself separate them: daily experience, motivation and selection into a study can differ along with sensory status. Years of Braille use, for example, are not randomly assigned. Comparing blind and sighted experts is more informative than comparing one expert group with novices, but expertise matching does not remove every difference in lifetime experience.

A controlled human study by Lotfi B. Merabet and colleagues used five days of complete blindfolding in normally sighted adults during intensive tactile training, with non-blindfolded training controls. Tactile stimulation increasingly recruited occipital cortex in the deprived group. Disrupting occipital function with repetitive transcranial magnetic stimulation impaired Braille-character discrimination in the blindfolded group on day five, but not in controls; the differential disruption was absent 24 hours after blindfold removal. The imaging response also reversed rapidly.^[ref] The imaging and stimulation arms used different participants, so one should not claim a within-person correlation between their measures.

This combination is more informative than cortical activation alone. Both groups received training, yet the deprived group temporarily depended on a different cortical contribution. Rapid reversibility is compatible with altered weighting or unmasking of existing connections; it does not require wholesale growth of new long-range pathways in five days. The result establishes a functional contribution under the tested state, not that occipital recruitment is sufficient for every tactile gain. Stimulation can affect a network, and the study cannot directly equate sudden reversible deprivation with lifelong blindness. Nor is the persistence of learned skill after the imaging effect disappears paradoxical: skill can remain while the supporting network changes.

A complementary causal result comes from Stephen G. Lomber and colleagues' experiments in congenitally deaf cats. Compared with hearing cats, these animals showed enhanced peripheral visual localization and motion detection on particular tasks. Reversible cooling of posterior auditory cortex removed the localization advantage, whereas cooling a dorsal auditory region removed the motion advantage.^[ref] Cooling provides an experimental contrast between a functioning and temporarily deactivated circuit. The task-specific double dissociation argues for functionally differentiated recruitment, rather than one undifferentiated increase in effort. The evidence is from cats and does not quantify the contribution of practice versus deprivation in humans.

A double dissociation is powerful but not magical. To interpret it, one must check that deactivation does not merely impair movement, alertness or basic visibility, and that tasks are not placed at incomparable difficulty levels. The existence of a recruited circuit identifies part of the implementation, not the entire developmental cause. Extensive experience could help establish that circuit's new role. Thus the strict opposition “neural reorganization or practice” confuses a mechanism with one possible source of the mechanism.

A discriminating program would measure performance before training, randomly assign normally sighted participants to matched training with or without temporary deprivation where appropriate, and follow learning and recovery. For established sensory loss, longitudinal observations and carefully matched comparison groups can constrain onset and experience effects without pretending that sensory loss can be freely randomized. Untrained tasks, different body sites or stimulus dimensions, delayed tests and speed–accuracy measurements distinguish broad transfer from repeated response memorization. Peripheral sensitivity should be checked before attributing every difference to cortex. These are design recommendations, not invented completed studies.

Measurement also needs a mechanistic bridge. Fitting psychometric functions can separate threshold and lapse changes; signal detection theory can distinguish sensitivity from a shifted decision criterion under explicit assumptions. Imaging identifies candidate recruitment, while causal disruption tests contribution. Neither activation magnitude nor a lower threshold alone establishes a “better” brain, and an adaptation useful for one task may create costs elsewhere. Residual input, age of onset and the sensory dimension previously calibrated by the missing modality can all constrain outcomes. Enhancement and impairment can coexist in the same person.

The evidence therefore supports an interaction among altered input, experience and circuit availability. Some compensation is task practice, some reflects state-dependent or developmental changes in neural recruitment, and the strongest explanation specifies how those factors cooperate rather than assigning every gain to only one.